The White House ratcheted up its criticism of Moscow Wednesday, pointedly saying that it had seen credible reports Moscow backed an election-day attack in Montenegro last year.

A senior administration official -- speaking as Secretary of State Rex Tillerson was in Moscow for difficult talks on Syria -- pointed the finger of blame at Russia for a coup attempt during October's election.

"We are very concerned about Russian interference in the October elections in Montenegro," the official said, "including credible reports of Russian support for an attempted election-day attack on the government."

A special prosecutor in Montenegro has accused Russian state bodies of trying to overthrow the government, in an effort to prevent Montenegro from entering NATO.

The comments also came a day after president Donald Trump approved Montenegro's accession to the alliance and prepared to meet NATO Secretary General Jens Stoltenberg at the White House later in the day.

Under Russian leader Vladimir Putin, the Kremlin has long seen the Balkans as within its "sphere of influence".

After pro-Russian campaign rhetoric, Mr Trump has tempered his praise of Mr Putin, most recently condemning his support for Syria's Bashar al-Assad.

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Mr Trump's harder line comes amid growing questions about whether his aides and associates colluded with Russian meddling in the US election. That line of inquiry has focused on Trump allies Carter Page, Paul Manafort and Roger Stone.